Hello, I have the same problem, and in fact, I realized that the "Favorite artists" section is visually limited and display only 100 artists (the last 100 artists you added). So when you reach this limit, if you add a new artist, it will replace the first of the last 100 artists visible in this section.
Please Apple, remove this limitation, because I want to be able to see all of my favorites artists 🙂
And it would be great also to be able to sort the list (recent / alphabetical..), and to change the layout.